Polymethyl methacrylate (PMMA) and polycarbonate have excellent transparency, and therefore have been widely used in the fields of, for example, optical materials, household appliances, OA machines, and components for automobile. PMMA, however, has a low heat deformation temperature, and is not suitable for uses that require high heat resistance, for example, optical components to be mounted on a car. Further, PMMA has high water absorption so that it is likely to suffer a change in size or a change in refractive index due to absorption of water, and thus PMMA is unsuitable for optical components. On the other hand, polycarbonate has excellent heat resistance and low water absorption properties. However, polycarbonate has a poor surface hardness, and further exhibits large birefringence due to the aromatic rings contained, and therefore the use of polycarbonate as an optical material is limited.
With respect to a resin having excellent heat resistance and excellent transparency, patent documents 1 and 2 have a description about a copolymer of methyl methacrylate, styrene, and α-methylstyrene. However, an α-methylstyrene copolymer has poor thermal stability such that it suffers heat deterioration during molding, causing a lowering of the mechanical strength. Further, in molding of the α-methylstyrene copolymer at a high temperature, decomposition product gas is generated to cause the appearance of the resultant molded article to be poor, and therefore the range of the molding temperatures is restricted.
Patent documents 3 and 4 have a description about a copolymer of methyl methacrylate, N-cyclohexylmaleimide, cyclohexyl methacrylate, and an aromatic vinyl compound (such as styrene or α-methylstyrene). However, when the copolymerization rate of maleimide is increased for improving the heat resistance, the resultant copolymer inevitably suffers discoloration, and therefore this copolymer is not suitable for the uses that require both high heat resistance and excellent transparency.
Patent document 5 has a description of a resin obtained by hydrogenating the aromatic double bonds of polystyrene. Polyvinyl cyclohexane obtained by hydrogenating polystyrene is a resin having excellent transparency and excellent heat resistance; however, the polyvinyl cyclohexane has a disadvantage in that the mechanical strength is poor.
Patent document 6 has a description of a resin obtained by hydrogenating 70% or more of the aromatic double bonds of a copolymer comprising (a) an (meth)acrylate and an aromatic vinyl monomer. This resin has excellent transparency and excellent heat resistance, but, for applying the resin to uses that require high heat resistance, for example, optical components to be mounted on a car, a further improvement of the heat resistance of the resin is needed. Further, in this patent document, as examples of the aromatic vinyl monomers, there are mentioned styrene, α-methylstyrene, and p-hydroxystyrene, but, in the working Examples of the patent document, only styrene is actually used as the aromatic vinyl monomer.
Patent document 7 has a description about a copolymer of methacrylic acid and styrene. In the invention described in this patent document, gel formation is suppressed by adding polyoxyethylene alkyl ether, and, when the copolymerization rate of methacrylic acid is increased for improving the copolymer in heat resistance, gel formation inevitably occurs. A gel causes the optical properties of the copolymer to be poor. Therefore, in the invention described in patent document 7, it is difficult to achieve the copolymer having both excellent optical properties and high heat resistance.